This invention relates to cathode ray tubes of the shadow mask type, and particularly to a system for suspending a shadow mask in such a tube.
In conventional color cathode ray tubes there is provided a shadow mask assembly including a heavy frame to which is welded a dished, apertured mask. The frame is, by design, extremely rigid and provides the necessary rigidity for the mask. The mask-frame assembly is mounted in a conventional tube by a suspension system comprising three or four leaf springs which are welded to the frame at spaced points around the periphery thereof. The springs have apertures at their distal ends which engage studs projecting inwardly from a rearward flange on the tube faceplate when the assembly is mounted in a tube. The mask-frame assembly is capable of being demounted and precisely remounted in a tube by depressing the springs to disengage the said studs. This type of system has proven to be commercially viable, however the mask-frame assembly is undesirably expensive.
A different type of shadow mask and suspension system therefor is disclosed in the patent to Fyler -- U.S. Pat. No. 2,961,560. This patent shows a frameless shadow mask supported at a multiplicity of spaced peripheral points directly by the concave screen-bearing surface of the tube faceplate. By this approach, the rigidity of the faceplate is used, in effect, to impart rigidity to the mask, thus eliminating the necessity for the mask to also be rigid. This approach, as disclosed by Fyler, however, suffers, inter alia, (1) from an undue difficulty and inconvenience in the demounting and remounting of the shadow mask in the tube, as is required by conventional screening practices, (2) a difficulty in seating the mask uniformly on the multiplicity of support elements provided on the faceplate, (3) uncontrollability of the spatial position of the mask corners, and thereby a loss of color purity in the corners of the displayed images, (4) a shifting of the geometrical center of the mask upon thermal expansion and contraction thereof, due to the non-equalized, frictional retention of the mask in the Fyler mask mounting system, (5) difficulty in achieving a commercially satisfactory "Q" compensation of the mask, and (6) its relatively high cost of manufacture and assembly.
The present invention represents a departure from these and other prior art approaches. The best features of both approaches discussed are abstracted without also acquiring the disadvantageous qualities thereof. By the present approach, a low cost, lightweight, torsionally flexible mask is provided. The faceplate is used to impart the necessary rigidity to the mask. However, rather than affixing the mask directly to the faceplate, a novel suspension system is provided which furnishes a mechanically rigid link between the faceplate and the mask, and yet which permits the mask to be conveniently and repeatably demounted and precisely remounted in the tube.
A system of the type taught by this invention has imposed upon it a number of requirements and constraints not presented in conventional systems in which a rigid frame is used to impart rigidity to the mask. Before enumerating these requirements and constraints, a discussion of certain principles underlying this invention will be engaged, A shallow mask of the type with which this invention is concerned may be modeled as a rectangular four bar linkage affixed to a stiff but flexible sheet. Such a model is shown in FIG. A. The four rigid bars of the linkage are designated A, B, C and D; the sheet is labeled S. As is well known, a four bar linkage is not inherently a rigid structure. The rectangular four bar linkage, in its free state, might, e.g., quite easily be skewed into a parallelogram geometry. It is evident, however, that the FIG. A model cannot be skewed in its plane to take a parallelogram shape since it is affixed to the sheet S.
The linkage can, however, be torsionally twisted about its diagonals, as shown for example in FIG. B. In FIG. B, the model has been twisted as follows -- the linkage bar A has been rotated toward the reader (see arrows); the linkage bar C has been rotated away from the reader. The corners 1 and 3 have been displaced upwardly and the corners 2 and 4 have been displaced downwardly. The sheet S is thus stressed convexly along diagonal 2-4 and somewhat concavely at the ends of diagonal 1-3. The model may thus be thought of as being twisted about one of its diagonals. It can be noted that the model configuration, after twisting, is changed substantially less along its major axis M.sub.a and minor axis M.sub.i than along the diagonal 2-4. Thus, a four bar linkage affixed to a flexible sheet is relatively stiff with respect to its major and minor axes (due to the rigidity of the bars), but is relatively flexible in torsion. When torsionally flexed (twisted) about its diagonals, the corners are displaced, but points on the major and minor axes remain relatively stationary.
As will be pointed out in more detail hereinafter, the lightweight shadow mask with which this invention is concerned closely corresponds to the described model in its mechanical characteristics.
The principles of this invention, though not limited to such application, are most useful when embodied in a color cathode ray tube having a flangeless faceplate. A flangeless faceplate, because of its lack of a stiffening flange, as found in conventional front panels, tends to vary in its shape from unit-to-unit. Specifically, flangeless faceplates will tend to vary (from unit-to-unit) in a twist-wise sense. Thus, one of the necessary requirements imposed on a mask intended for use with a flangeless faceplate is that it must be able to adapt to such twist-wise, tolerance-related deviations in a faceplate with which it is mated. Stated another way, the mask may be capable of flexing or twisting about its diagonals in the same way faceplates are apt to twist-wise vary from unit-to-unit in their contour. As will become evident as this description proceeds, the shadow mask with which this invention is concerned is uniquely capable of meeting this requirement.
Secondly, and of equal significance -- with respect to any given faceplate, since the mask is non-self-rigid, the suspension system for the mask must effectively transfer the rigidity of the faceplate to the mask. The suspension system must also fix the spatial position relative to the inner surface of the faceplate in spite of repeated demountings and remountings thereof, and in spite of mechanical configuration shifts resulting from stress relieving of the mask and appurtenant structures. Further, the suspension system must be capable of substantially immobilizing the geometrical center of the mask upon thermal expansion and contraction thereof in order to prevent temperature-related color degradation in the reproduced pictures.
The afore-discussed Fyler (U.S. Pat. No. 2,961,560) mask suspension system, in addition to suffering from a mounting-demounting inconvenience and the other drawbacks described, demounting and remounting thereof during screening, suffers also from the manner in which it causes the mask to be suspended. The Fyler system utilizes six or more off-corner mask support points aand three mask indexing points (one on the mask minor axis and the other two in opposite corners of the mask). It is noted above in connection with the discussion of FIG. B, that it is the corners of a torsionally flexible mask which are uncontrollable, while points on the major and minor axes tend to remain relatively stationary. The Fyler patent fails to disclose mask suspension points at all corners of the mask. As will be easily understood from a consideration of FIGS. A and B, a suspension system of this nature does not control or arrest the corners of the mask adjacent that point, as it must. The result is that a suspension system of the Fyler type will be apt to have color purity errors, especially in the corners.
Further, because of the inherent (by design) non-self-rigid nature of the mask with which this invention is concerned, it may be easily distorted by excessive or improperly directed forces applied thereto. Also, such a mask is apt to undergo configurational changes during stress relief operations, and is otherwise easily influenced in its shape by forces exerted thereon. It is also desirable in a mask suspension system of the type taught by this invention that no force components, especially tangential force components, be imparted to the mask in the plane of the mask which might tend to rotate, mislocate, twist or distort the mask.
The present invention provides a detachable spring suspension system by which a non-self-rigid shadow mask is supported at its four corners. The teachings of the prior art, including the discussed Fyler patent would be totally inadequate to provide a commercially viable system for supporting a non-self-rigid mask of the type with which this invention is concerned. Because the structure and mounting requirements of this novel mask are so radically different from the prior art mask structures and mounting requirements, the mask suspension systems which have been developed for use in prior art tubes are not suitable for suspending the described novel mask.